CD27+ B cells in human lymphatic organs: re-evaluating the splenic marginal zone

The marginal zone of human spleens is regarded as an organ-specific region harbouring sessile memory B cells. This opinion has arisen by extrapolating from results obtained in mice and rats. Detection of CD27(+) B cells in situ now revealed similarities among the most superficial region of B-cell follicles in human spleens, reactive lymph nodes, inflamed appendices, tonsils and terminal ilea. The follicular surface in these organs consists of small naïve immunoglobulin D (IgD)(+) CD27(-) B cells predominating in an inner area and larger IgD(+/-) CD27(+) B cells prevailing in a more superficial position. CD27(+) B cells may, however, also occupy the entire follicular periphery around the germinal centre. Together with additional peculiarities this distribution indicates a fundamental microanatomical difference among the human and rodent splenic white pulp. We hypothesize that the follicular periphery represents a recirculation compartment both for naïve and memory/natural reactive B cells in all human secondary lymphatic organs. This assumption implies a difference in recirculation behaviour among human and rodent B memory cells.